This invention relates to strip materials that have headed projections which will releasably engage so that two severed portions of the strip materials will provide a releasable fastener between different objects.
U.S Pat. No. 4,290,174 describes such a strip material which comprises a flexible polymeric bonding layer; a multiplicity of flexible, resilient, generally U-shaped monofilaments of polymeric material, each including a central bight portion embedded in the bonding layer in a rectangular array, two stem portions extending from the bight portion and projecting generally normal to a surface of the bonding layer; and enlarged, generally circular heads at the distal ends of the stem portions. Each of the heads has an outer cam surface adapted for engagement with the cam surfaces of heads along a different portion of the strip material to produce deflection of the stem portions and movement of the heads on the stem portions past each other to releasably engage the portions, and has a latching surface opposite the cam surface, which latching surface is generally planar, extends at generally a right angle radially from its supporting stem portion, and is adapted to engage similar latching surfaces on the heads of the other portion when the portions are engaged.
While fasteners made from two portions cut from the strip material described in U.S. Pat. No. 4,290,174 have provided many advantages over other known fasteners for many applications, that strip material can not provide both a moderate level of engagement and disengagement forces between portions of the strip material while restricting unwanted relative movement between the engaged portions in response to the application of forces applied to the two fastener portions in a direction parallel to their backing layers and to the rows. If the stem portions and heads are so closely spaced that the rows of headed projections on one portion can not slide between the rows of headed projections on the other portion under these conditions, the force required to engage and disengage the portions is so high that a fastener made using the portions is not useful for many purposes. If the stem portions and heads are spaced sufficiently that the force required to engage and disengage the portions is at the moderate and often more useful level, the rows of headed projections on one portion can slide between the rows of headed projections on the other portion, thereby allowing the portions to become partially or totally disengaged. U.S Pat. No. 4,290,174 describes reducing this problem by (1) varying the spacings of the stems along the rows extending longitudinally of the strip so that at least when the rows of two articles with such varied spacing are engaged at right angles to each other, greater separating and shear strengths will be developed, or (2) disposing the rows of U-shaped filaments so that their stems are not aligned normal to or parallel with the edges of the strip so that when a user engages two portions of the strip with their edges parallel (as he would normally be expected to do), the rows on the articles will cross each other to develop the maximum strength in the fastener both in tension and shear, or (3) shifting successive rows of U-shaped filaments slightly in a direction transverse to the strip so that the stems of successive rows will not be aligned and thus will not permit shearing longitudinally or transversely of the strip.
While these techniques would help reduce the problem, they do not preclude slippage between the rows for all possible orientations of the rows. Additionally, the mechanism for accomplishing the second and particularly the third technique mentioned complicate the device on which the strip material is produced more than might otherwise be desired, particularly when wide widths (i.e., 6 inch or greater widths) of the strip material are made.
Thus, the solution to the problem described in U.S. Pat. No. 4,322,875 was developed, which involves utilizing two different strip materials with rectangular arrays of headed stems, each of which strip materials has stem portions that are about equally spaced in each direction to provide numbers of stem portions per unit length along the surface of its bonding layer in each direction that are different from and not a multiple of or evenly divisible by the number of stem portions per unit length on the other strip material in either direction (e.g., 20 stem portions per inch in each direction on one strip material, and 12 stem portions per inch in one direction and 14 stem portions per inch in the other direction on the other strip material). With this construction, a desired useful level of engagement and disengagement forces can be provided for portions of the strip materials and when portions of the different two strip materials are engaged with rows aligned, certain of the stem portions will always interfere with each other to restrict relative movement between the portions of the strip materials in a direction parallel to their bonding layers. While this solution is reasonably effective and has been used commercially, it requires manufacturing and stocking two different strip materials.